The uses of materials in aircraft gas turbine engines have become increasingly demanding in recent years. The requirements of increased performance and decreased fuel consumption place a premium on high strength and light weight. Improved performance generally relates to increases in operating temperature, so that material strengths must be retained at higher temperatures than previously encountered.
Titanium alloys generally provide high strength with light weight, although their useful strength is limited to approximately 1000.degree. F., and special precautions must generally be taken to prevent oxidation. Titanium aluminides, generally of the TiAl or Ti.sub.3 Al type, retain useful properties up to about 1500.degree. F., but their usefulness is limited because their low temperature ductility greatly limits the fabrication techniques which may be used, and makes them highly susceptible to matrix cracking due to mechanical damage incurred during normal handling and usage at ambient temperature.
It is well known to increase the strength of structural materials by embedding high strength fibers in a matrix material to form composite materials. While these composite materials generally benefit by combining the best properties of the component materials, such as the high strength of the reinforcing fibers, they can also be limited by other properties of the materials.
Titanium alloy fiber reinforced composites have improved strengths, but are still limited by the high temperature strength and low oxidation resistance above 1000.degree. F. Titanium aluminide matrix fiber reinforced composites also have improved strength, with the improvements being retained up to 1500.degree. F. Fabricability of the titanium aluminide fiber reinforced composites is very limited because of the low room temperature ductility of the titanium aluminide.
In U.S. Pat. No. 4,816,347, to Rosenthal, et al., this limitation of low room temperature ductility was overcome by interposing layers of a titanium alloy having good ductility, positioned to surround the high strength reinforcing fibers, between sheets of titanium aluminide, thus providing a hybrid titanium metal matrix composite having good strengths at temperatures up to about 1500.degree. F. and good room temperature mechanical properties including good ductility and improved resistance to matrix cracking.
The improved high temperature strength of a titanium aluminide matrix fiber reinforced composite material is generally accompanied by limited fabricability due to low room temperature ductility. Rosenthal, et al., were able to resolve this problem only by the addition of a lower strength titanium alloy material, thus forming a hybrid composite. The addition of the lower strength material, however, results in a reduction in the overall capabilities of the composite.
Siemers, in U.S. Pat. No. 4,786,566, disclosed a method for formation of a fiber reinforced trititanium aluminide matrix composite which involves plasma spraying of the matrix material onto an array of aligned fibers to form a fiber reinforced sheet. The sheets are then laid up and bonded together to form a fiber reinforced object. Siemers reported that the composites had good strength, but that the ductility was somewhat limited. This technique avoids the difficulties associated with trying to form thin sheets of the low ductility titanium aluminide material, but does not provide composites which are particularly usable.
Composites made without the ductile matrix suffer performance deficits during such tests as thermal fatigue cycling, where the component is exposed to temperatures ranging, generally, from room temperature to an elevated service temperature. Large stresses are generated in the boundary region at the interface between the fibers and the matrix, due to the large mismatch in the thermal expansion coefficients of the reinforcing fibers (2.7.times.10.sup.-6 /.degree.F. for SCS-6 silicon carbide fibers, a product of Textron Specialty Metals/Subsidiary of Textron, Inc.) and the matrix material (5.7.times.10.sup.-6 /.degree.F. for Ti.sub.3 Al. These stresses frequently cause cracking in the matrix, and/or disbonding of the reinforcing fibers from the matrix, which leads directly to failure of the composite.
Thus, what is needed is a material which achieves the good high temperature strength properties of a titanium aluminide matrix fiber reinforced composite material while retaining good low temperature ductility.